The present invention relates to a drill unit with a drill bit having a cooling channel, and with a device for supplying cooling fluid to the cooling channel of the drill.
The engineering industry has at all times aimed at productivity increases and quality improvements. For accomplishing this at hole cutting operations, which constitute one of the most difficult operations within the engineering industry, one usually aims at increasing the cutting speed of the drill in order to in this way obtain a higher productivity. A higher cutting speed also gives a more finished surface, since the burr formation decreases. A problem with increased cutting speed, in particular when drilling in metal, is however, that a strong wear of the corners of the drill is obtained, due to the high temperature coming up at the machining. For decreasing the wear, cooling liquid is supplied, recently by means of cooling channels shaped in the drill. An example of such a drill is shown in U.S. Pat. No. 5,173,014, which describes a drill with two complete teeth, two peripherally working teeth and two cooling channels. By using a number of teeth, a more advantageous power distribution of the drill is obtained, which increases the precision of the drill. U.S. Pat. No. 5,174,691 shows a long drill with a central cooling channel, which is completed with two openings. For enabling a good chip transport at long drilled holes, the cooling liquid is supplied under a pressure of about 5500 kPa.
It has also been shown to be difficult achieving holes with required tight tolerances using conventional drilling, and the feeding of the drill must often be restricted due to the fact that high axial forces are required. An available method providing good tolerances and requiring low axial force is gun drilling. However, in this method a good control of the drill is required and small feedings, which gives a low manufacturing rate. These requirements lead to the result that it is often not possible to use this method, but rather traditional drilling is used with after-treatment with a broach. Moreover, in many cases the holes must be reamed.
The present invention relates to achieving a drill unit, which is not afflicted with the drawbacks mentioned above, and which admits a high productivity without renouncing the quality of the holes and with a long lifetime of the drill.
The purpose of the invention is obtained by means of a drill unit with a drill having a cooling channel, and with a device for supplying cooling fluid to the cooling channel of the drill, characterized in that the drill comprises three teeth, of which the first tooth extends from the periphery of the drill to its center, while the second and the third teeth extend inward from the periphery of the drill without reaching its center, that the cooling channel extends axially in the center of the drill and is deflected in the bit point of the drill so that it emerges in the space between the center and the terminal points of the second and the third teeth, as well as the supply arrangement for the cooling fluid is applied for supplying the fluid under high pressure. By shaping the drill with three teeth, a high productivity is enabled with tight tolerances of the drilled holes at the same time as only a small axial feeding power is required, as only one of the teeth works in the center of the hole. By means of supplying the cooling medium under high pressure a more effective cooling is obtained, which leads to a small wear of the drill and a lesser risk for structural changes in the machined material. At the same time the chip transport is facilitated providing a decreased risk for chip stopping, tool breakdown and damage to the hole surfaces. As the cooling channel extends substantially in the center of the drill, a maximum rigidity of the drill is obtained.
In a preferred embodiment the pressure of the cooling fluid supplied is larger than 3000 kPa and the teeth are unequally distributed around the circumference of the drill, whereby the angle, seen a circumferential way between the first tooth and the second and the third teeth, respectively, are 110xc2x0 and 200xc2x0, respectively. Further, partially conical slots are shaped in the drill core between the three teeth, which slots extend toward the center of the drill. The partially conical slots are preferably shaped using a tool consisting of a conical grinding tool, the generatrix of which at the machining of the drill core forms an angle of at least 30xc2x0 to the drill axis.
A most significant difference resulting from having only one tooth extending to the center of rotation relative to the situation when two teeth are allowed to extend to the center is that the drill contains no chisel edge and its negative cutting geometry which rather ploughs than cuts. Due to the fact that no chisel edge exists, the power picture is changed so that weaker workpieces deflect less in the longitudinal direction of the hole, which leads to better tolerances.
For prior art drills having a large chisel edge. 20% of the drilling of the hole diameter (i.e., the center of the hole) requires 80% of the axial force. A small part of the axial force reduction can be recovered by using high cooling fluid pressures.
When the angle of clearance from a geometrical point of view cannot be made sufficiently large on prior art drills having two or several teeth extending to the center, it is evident from the geometry that this problem is considerably better solved having only one tooth extending to the center. This is done for eliminating the negative impact the angle of clearance has in the center.
According to our judgement the invention according to U.S. Pat. No. 5,173,014 has not solved the problem with a high feeding power in the axial way, which is the consequence with having a plurality of teeth extending to the center rather than having only one tooth extending to the center. Only one tooth extending to the center, makes it possible to obtain positive cutting angles at the center.
An additional aspect is that it is considerably more difficult to achieve four points to coincide on a circle than three points, the latter resulting in a better cylindricality of the complete hole due to a disadvantageous power picture, with differentiated angles and in the present case there is a possibility to reduce the reduce the risk for self-generated torsional oscillations. Further a drill with four teeth has a lesser chip space leading to increased chip problems.